It has become evident that nitrogen and phosphorus affect the productivity of aquatic life - nitrogen being the limiting nutrient in eutrophic waters and phosphorus in oligotrophic waters.
It is also known that municipal waste waters contain from 10 to 100 times the nutrient concentration of agricultural or forest drainage. Consequently in densely populated areas overfertilization of surface waters, algae growth and chemical pollution have been noticed along with degradation of potable water sources and the need for more efficient removal of organics, nitrogenous compounds and phosphorus from all waste waters has become more obvious.
While removal of phosphorus can be achieved reliably by chemical precipitation and removal of organics by biological followed by physical chemical processes, removal of nitrogen is more complicated.
The current processes of ammonia stripping, break point chlorination, ion exchange and biological nitrification followed by biological denitrification are relatively complex and expensive.
Although conventional activated sludge process is capable of removing up to 50-60 percent of the nitrogen from treated waste waters, the removal of nitrogen is eratic and in most cases its major part is discharged in mineralized form in the effluent.
The biological transformations of nitrogenous compounds occur through biological processes of ammonification, nitrification and denitrification. The first two require presence of oxygen, the denitrification require absence of oxygen. The two distinct mechanisms responsible for removal of nitrogen and reduction of nitrite and nitrate are (a)--formation of ammonia followed by transfer of ammonia into the anabolic cell metabolism which is of minor importance since C:N ratio in cell tissue is 5-6, whereas in domestic waste waters only 2-2.5, and (b)--microbial denitrification, which is a respiratory reduction in which nitrite and nitrate replace oxygen as the final electron acceptors in the respiration chain.
Considerable number of heterotrophic facultative and obligate anaerobic bacteria species can transfer electrons to nitrite instead of oxygen in the respiratory chain, and all substrates normally used by the cells for respiration may serve as electron donors for the reduction process regardless of whether they are present in the external medium, or whether they have to be mobilized from intracellular assimilates.
The reduction of nitrate is considered to be an adaptive property of those bacteria that can reduce nitrite for respiration.
The reduction mechanism for biooxidation of organic matter and for transformation of the nitrogenous compounds is expressed by reactions listed in Table 1.
Biooxidation of organic matter, ammonification and nitrification cause no problems. Nitrification is efficient and reliable if organic loadings are maintained below 0.3 lb BOD per lb of mixed liquor suspended solids per day, sludge age is maintained above 3-4 days and disolved oxygen in the mixed liquor is maintained above 1.0-2.0 mg/lit. The higher is the concentration of mixed liquor suspended solids, the more efficient is the transformation of ammonia to nitrite and nitrate. The overall rate of denitrification is a function of the concentration of the heterotrophic facultative bacteria present in mixed liquor suspended solids and their activity in the absence of oxygen. To maintain the activity the denitrifying bacteria must be supplied with suitable organic material-source of energy.
From the process point of view to accomodate the different food and oxygen requirements of biooxidation of organic matter, ammonification and nitrification of nitrogenous compounds and decomposition of nitrite and nitrate by the microbial respiration and to achieve acceptable reaction rates and efficiencies, the various biochemical reactions are currently being carried out in separate process stages. While the conventional activated sludge process is capable of removing nitrogen with efficiency of 50-60 percent, the current multistage processes can achieve the removal of nitrogen with efficiency up to 80-90 percent.
The multistage processes currently in use in large municipal treatment plants require treatment facilities that are too complex to be scaled down to small package plants to serve small developments or single family dwellings. More, the established biological processes are inefficient in removal of biologically resistant compounds recognized as refractory organics and the Physical-chemical treatments are inefficient in removal of low organic acids and low carbohydrates both highly soluble and present in considerable concentrations in domestic and municipal waste waters. Consequently neither the biological, nor the chemical - physical treatments alone can efficiently reduce the concentration of organic contaminants to the level found in fresh waters. The required combination of the biological and physical - chemical treatments therefore further increases the complexity of the treatment and the treatment facilities.
Furthermore, biological processes that utilize low organic loadings to achieve low yields generate sludges that have poor coagulation properties, requiring low overflow rates in clarifiers and causing relatively high losses of solids in effluents.
Because of the number of process steps required for removal of the various pollutants renovation and reuse of domestic waste waters at present seems feasible only if practiced on a large scale.
A large waste water treatment--renovation plant however requires large underground sewage collection--transportation network, and in case the renovated water is reused a large underground water distribution network. Such networks are expensive to build and even more expensive to maintain and to operate.
It is therefore obvious that on-site renovation and reuse of waste water would be economically more attractive than renovation and reuse of waste waters via central collection-treatment-distribution. However, because of the complexity of the involved treatment on-site renovation and reuse of domestic waste water as yet can't be practiced.
Although, at present small package sewage treatment plants are available, they do not provide the required degree of purification that would permit reuse of the purified effluent and in general they also lack the required process stability.
It is therefore the prime object of this invention to reduce the number of process steps required by the present art and carried out in separate process stages when removing suspended solids, carbonaceous material, nitrogenous material, phosphorus, bacteria and viruses from domestic waste waters and to provide a purification process having a simple process sequence and capable of removing the various pollutants more-less simultaneously.
More particularly it is the object of this invention to provide a process for purification of sanitary waters and purification of some industrial waste waters, which would be capable of achieving reliable and efficient removal of suspended solids, biooxidation of organic matter, biological nitrificat on, biological denitrification, chemical precipitation of phosphorus, chemical oxidation of refractory organics and toxic compounds, efficient kill of bacteria and viruses and removal of the unreacted oxidizing agents to render the purified waste water suitable for reuse.
Another object of this invention is to provide a process for purification of waste waters, that will not be affected by shock hydraulic and organic loadings.
Another object of this invention is to provide a prosess for purification of waste waters, that will produce minimum excess sludge, will require minimum energy and will be capable of unattended operation.
Other objects and features of the invention will be set forth fully here in after.
The full nature of the invention will be understood from the accompanying drawings and following description and claims.